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Dawn at Ceres Ceres from Dawn’s Data M.C. De Sanctis Istituto di Astrofisica e Planetologia Spaziali – INAF Rome, Italy [email protected] Ceres - The Basics • 482 x 482 x 446 km • mean radius 470 km • Rotation period 9.074 hr • Ceres’ surface reflects <10% of incident sunlight • Average surface temperature 110- 155K-Maximum at equator-subsolar point ~230-240 K • Density 2.162 kg m-3 • Ceres as a whole is ~50 vol.% water • Early models suggested Ceres could have a 50-100 km thick ice shell NASA/JPL-Caltech/UCLA/MPS/DLR/IDA Road Map to Vesta and Ceres Ceres is the first ice-rich body subject to extensive mapping for Vesta Departure geology, mineralogy, elemental (2012) composition, and geophysics Earth Dawn launch (2007) Sun Vesta Arrival (2011) Ceres Arrival (March 2015) Dawn Instruments + Radio Antenna Camera Gamma Ray and Visible and Infrared Neutron Mapping Spectrometers Provided and Spectrometers operated by the Provided by the Italian Space German Aerospace Provided by Los Alamos Agency and the Italian National Agency and the Max National Labs and operated Institute for Astrophysics, and Planck Institute for by the Planetary Science operated by the Italian Institute Solar System Institute for Space Astrophysics and Research Planetology Why Ceres ? • The early asteroid belt may have been scoured by icy bodies, scattered by the formation of the remaining gas giants. • Today only some of the largest asteroids remain relatively undisrupted, and Ceres has a very primitive surface, water-bearing minerals, and possibly a very weak atmosphere and frost. 5 Ceres’ Peer Group: Icy Moon and Dwarf Planets • Ceres is expected to have water and ice in its interior, but more rock than the icy moons Enceladus and Dione. It has been classified as a dwarf planet by the IAU, like Pluto. 6 Outline Observational Constraints Open Questions Context to keep in mind Mixture of rock and ice Which rocks? Which ices? Function of origin Context to keep in mind Heat sources? Relative movements ? Chemical fractionation? Function of parent-body processes Observational Constraints Dawn Revealed an Evolved and Volatile-Rich New World! C B A D A- Oceanic materials make up bright Ceres shows topographic and high low deposits in Occator craters B- Signature of volatile takes the C- A large landslide reveals water ice D- The emplacement of 4- form of pitted terrains in Ikapati at Oxo crater km high Ahuna mons crater requires a partially molten source Ice Castillo Rogez, 2017 Properties of the outer shell? • Ceres has +7.2 to -6.4 km of topography relative to geoid • Gravity data indicate Ceres is close to hydrostatic equilibrium (Park et al., Nature, 2016) Rheology of the outer shell? Largest craters are ‘missing’ (Marchi et al., Nature Comm., 2016) Rheology Largest observed craters can be fully relaxed (but not the case for smaller craters) (Bland et al., Nature Geoscience, 2016) Evidence for a rheologically ‘weak’ outer shell at the 100km scale (Marchi et al., Nature Comm., 2016) (Bland et al., Nature Geoscience, 2016) Water in the outer shell? Some indications Transition observed from simple to complex craters (Hiesinger et al., Science, 2016) Water in the outer shell? Some indications Transition compatible with those observed on « icy » bodies (Hiesinger et al., Science, 2016) Ceres’ Surface Shows Evidence for Mineralogy Formed at Depth Sodium carbonates Ammonium Salts De Sanctis et al. (2016) Enceladus Global, Homogenous Composition: Ammoniated clays, serpentine, carbonates (De Sanctis et al. 2015), Ammannito et al. (2016) Ceres’ Average Surface Composition Suggests Advanced Aqueous Alteration • Distinct absorptions bands at : OH-stretching vibrations (Mg- phyllosilicates) : very broad - overlapping absorptions of several species (De Sanctis et al., Nature, 2015) : Carbonate Composition is evidence for band-Mg-Ca carbonate aqueous alteration De Sanctis et al., Nature under review 2.7 micron band center map Band Center maps at 2.7 and 3.1 microns show ubiquitous presence of phyllosilicates. The position indicate Mg- phases, like Mg-serpentine or Mg-smectite 3.1 micron band depth map Band Depth maps show local variations in proportions of ammonia-rich and Mg-rich clays. Such variations are likely due to spatial variability in relative mineral abundance. The widespread presence of these minerals is a strong indication of a global and extensive (Ammannito et al., Science, 2016) aqueous alteration. CM/CI carbonaceous chondrites still provide the best compositional analogy for Ceres, except for the NH4-clays Less altered More altered clays Clay minerals form in the most heavily altered CM/CIs Takir et al. (2013) Ammoniated Clays Are Strong Diagnostic of Environmental Conditions • Requires high partial pressure of hydrogen • Consistent with previous modeling of large ice-rich bodies (e.g., McKinnon and Zolensky 2003) • Ammonium exchanges with Na, Ca, K – Abundant salt production • Implies temperatures < 50 deg. C T=15o (Neveu and Desch 2015) . Castillo-Rogez et al , 47th LPSC22 Carbonates Help Narrow Down the redox-pH Space • Evidence for accretion of CO2 and/or organics • Ca/Mg carbonate formed with rock as part of global hydrothermal event • Other type of carbonate: NaHCO3 condenses upon / dolomite freezing of original ocean T=15T=15oo Castillo Rogez, 2017 Occator Bright Deposits are Rich in Carbonates (De Sanctis et al., Nature, 2016) N N • Largest carbonate deposit known outside Earth Infrared absorptions • Formed in alkaline within Occator crater aqueous environment correlate with brightness • Similar to Enceladus indicates concentration plume chemistry of Na-carbonate Na-carbonate is the major species (45-80 vol.%) In addition there is ammonium carbonate or chloride AhunaLocal Mons variations: Ahuna Mons • Ahuna Mons appears to be a viscous cryvolcanic dome. • The extrusion of the dome is recent (Ruesch et al., Science, 2016). 21x13 km, 4 km high The spectra indicate the Ascending subsurface fluids? presence of Na-carbonate as in Occator. (Zambon et al., 2016) Ceres’ Subsurface is Enriched in Salts • Sodium bicarbonate and ammonium salts found in bright deposits at Occator and other places (De Sanctis et al. 2016) • The emplacement of Ahuna Mons implies the presence of brines at depth (Ruesch et al. 2016) • Occurrence of salts in many settings suggest near-surface abundance (Stein et al. 2016) GRaND Grand H count Neutron counts drop towards the poles Evidence for water ice at high latitudes (at depth <1m) (Prettyman et al., 2016) Direct evidence for water in the outer shell • Reflectance spectra VIR reveal the presence of exposed surficial H2O on Ceres (Combe et al., Oxo H2O Science, 2016). • There are other locations where VIR data of H2O water ice is exposedrich area in Oxo on the surface Chemical Fractionation Points to Advanced Hydrothermal Activity Cb? Ch? Ceres After Young et al. (2003) Mineralogy points to global episode of hydrothermal activity in Ceres’ early history • Large scale hydrothermal circulation supported by GRaND data (Prettyman et al. 2016) • Activity fueled by 26Al decay heat ? - Formation within a few My after CAIs • Water-to-rock ratio is > 2 (Castillo- Rogez et al., in prep) - Release of huge amounts of H2 - Leaching of soluble elements from rock, yields ~5 wt.% salinity (average) After Young et al. (2003) Castillo Rogez, 2017 Ice Shell Formation is the Expected Outcome of Differentiation in Large Ice-Rich Bodies Freezing front Freezing • Large rock particles sink, fines stay in the ocean Ice (Kirk and Stevenson 1987) – Flocculation also promotes clay sedimentation Ocea n • Slow freezing over ocean leads to mostly pure Muddy/Salty ice shell, except for soluble salts at grain Ocean boundaries Mud – Rock particles cannot be accommodated at ice grain boundaries • There is no evidence for silicates on Enceladus, Dione, Europa, etc. Rocky CoreCore with large rock particles Castillo Rogez, 2017 But Geophysical Data suggest Ice is not a Dominant Component of the Shell •Crater morphology indicates ice content <40% (Bland et al. 2016) Ice/rock/ •Gravity suggests partially salt shell differentiated interior (Park et al. 2016) – Shell density ranges from 1.1 to Rocky 2.0 g/cm3 core •Topography can be explained by strong shell (40-50 km) over soft interior (Fu et al. 2016) – Shell density narrowed down to 1.35 - 1.65 g/cm3 • Ceres could have lost its ice shell (impacts? accelerated sublimation? ) Castillo Rogez, 2017 Modeling Ceres Gravity Shape Density Original Thermal Profile Composition in Modeling Volatiles Rock Equilibrium* Composition Original Rock Chemistry Core Composition Liquid Cracking Composition Temperature FREZCHEM** Elemental Shell Composition Structure Mineralogy Crater Morphology Impact History * Geochemist’s Workbench by Bethke – also works for non-equilibrium chemistry ** Marion and Mironenko (2015), freely available Castillo Rogez, 2017 Models results • Ceres’ Surface Composition Requires an original Water/Rock > 2 • Magnetite is the dominant form of the final iron – Most abundant mineral with the clays – Iron also captured in the form of sulfide, about three times less (molality) than magnetite • Sulfates are not expected • Chloride salts are expected • Potassium is leached from the silicates way before equilibrium is reached Science questions at Ceres • Where did Ceres form? • Is it physically/chemically differentiated? • Where’s the water and what role did it play? • Can there be Meteorites from Ceres ? • Is Ceres of astro-biological interest ? Where did Ceres Form? Ceres may have formed in the trans-Neptunian
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